This invention relates to a process to convert alkyl-substituted aromatic compounds using a catalyst formed from crystalline borosilicate AMS-1B and more particularly relates to a method of isomerizing xylenes and converting ethylbenzene in which a desired product mix is obtained.
U.S. Pat. Nos. 4,268,420 and 4,269,813, incorporated by reference herein, disclose a use of crystalline borosilicate AMS-1B in a catalyst which simultaneously can isomerize xylenes and convert ethylbenzene. In commercial manufacture of para-xylene, typically a feedstream containing C.sub.8 aromatics (p-xylene, o-xylene, m-xylene and ethylbenzene) is used. From such stream p-xylene is removed, typically by crystallization or absorption, and the remaining mixture is contacted with a catalyst which isomerizes o-xylene and m-xylene to a mixture containing approximately a thermodynamic concentration of p-xylene. The isomerized mixture is recycled to the p-xylene removal unit. Within the process are units which remove by-products including fuel gases (such as ethane), benzene, toluene and heavy aromatics such as diethylbenzenes. Because of the difficulty of removing ethylbenzene from xylenes by distillation due to closeness of boiling points, it is desirable to convert ethylbenzene to other aromatic species by hydrodeethylation which mainly produces ethane and benzene or disproportionation/transethylation which mainly produces benzene, diethylbenzenes and dimethylethylbenzenes. A commercially useful catalyst for the isomerization of xylenes typically converts ethylbenzene.
It has been found that crystalline borosilicate AMS-1B formulated as a catalyst including an alumina matrix material and a catalytically-active metal such as molybdenum predominantly converts ethylbenzene through hydrodeethylation. Depending on the economic value of individual by-products from a paraxylene manufacturing unit, and the composition of the aromatics feed stream, sometimes it is advantageous to convert more ethylbenzene through a disproportionation/transethylation method. A process which permits a single catalyst composition to convert ethylbenzene by either the disproportionation/transethylation or hydrodeethylation methods would be useful. A process which permits rapid change from one conversion method to the other would be very advantageous.